1. Field of the Invention
The present invention relates to a color image printing system. It relates particularly, though not exclusively, to a system including a color image forming apparatus that sequentially transfers images of colors formed on multiple image carriers that are arranged side by side to a storage medium that is being fed to form a color image, and a color-image-data generating apparatus that supplies data to the color image forming apparatus.
2. Description of the Related Art
Color image forming apparatuses, such as printers adopting electrophotography, generally employ a method of forming a full-color image by developing images of colors using multiple developing units for one photosensitive member, repeating an exposure-development-transfer process several times to superimpose the images of different colors on one sheet of transfer paper in order to form a color image, and fixing the color image.
Japanese Patent Laid-Open No. 11-98343 discloses a method of transferring compressed data to a printer of the above type, decompressing the data, and supplying the decompressed data to the printer engine.
It is known that an image can be distorted depending on the manufacturing accuracy of the exposing unit in such a printer to cause a color shift. In order to prevent the color shift, methods are proposed in which a test toner image is formed on a sheet of transfer paper or on a carrying belt in a transfer unit, and the test toner image is detected to correct the optical path of each optical system or to correct the writing start position of the image of each color on the basis of the detection result (for example, refer to Japanese Patent Laid-Open No. 64-40956 and Japanese Patent Laid-Open No. 2000-177170).
Japanese Patent Laid-Open No. 8-85237 discloses an apparatus that automatically converts the coordinate position where the image data of each color is output into the coordinate position of which any registration error is corrected. In this apparatus, a correcting unit corrects the position of a modulated light beam in units smaller than the minimum dot unit of a color signal on the basis of the image data of each color whose coordinate position is converted.
In order to correct the optical path of an optical system, it is necessary to mechanically operate the correction optical system (including a light source and an f-θ lens) and mirrors in the optical path to align the positions of the test toner images. For this alignment, a more precise movable member is required, thus increasing the cost of the apparatus. In addition, since it takes a long time to complete the correction, it is impossible to frequently perform the correction. Furthermore, any shift in the length of the optical path can be varied with time due to a variation in the temperature of the apparatus. In such a case, it is difficult to prevent any color shift by correcting the optical path of the optical system.
In the correction of the writing start position of an image, there is a problem in that it is not possible to correct any tilt of the optical system or to correct any magnification error caused by a shift in the length of the optical path, although any positional shift at the left end and at the upper left corner can be corrected.
In the apparatus disclosed in Japanese Patent Laid-Open No. 8-85237, there is a problem in that the correction of the coordinate position where the image data of each color is output in an image that has been subjected to halftoning can degrade the reproducibility of the dots in the halftone image to cause unevenness in color and to make Moire patterns obvious. FIG. 4 illustrates an example of the unevenness in color. Referring to FIG. 4, an input image 101 has a uniform density. When an image 102 given by performing color-shift correction to the input image 101 is printed, a linear relationship is not established between the value of the input image 101 and a toner density 103. Accordingly, an image having uneven densities is printed after the color-shift correction although the input image 101 has a uniform density. If such uneven densities are periodically repeated, there are problems in that Moire patterns are made obvious and, therefore, fine color images are not generated.
A photosensitive member which laser beams scan does not stop but moves by a predetermined amount corresponding to the printing conditions during the scanning by the laser beams with the increasing printing speed. If the laser beams of different colors have the same scanning direction, any tilt of the scanning line caused by the movement of the photosensitive member produces no problem. However, if the laser beams of different colors have different scanning directions, an unevenness in color can be caused depending on the amount of the movement of the photosensitive member. The amount of the movement of the photosensitive member can vary depending on the conditions of the printing medium and, therefore, it is not possible to correct the unevenness in color by a single process.
In the methods disclosed in Japanese Patent Laid-Open No. 64-40956 and Japanese Patent Laid-Open No. 2000-177170, all the data transmitted from the hosts is received and, then, the reading position of the data is adjusted in accordance with the bend characteristics of an apparatus that outputs the data to reproduce the image data. In high-speed printing using these methods, it is necessary to store all the images in a memory and to adjust any time lag of the photosensitive drum. Accordingly, it becomes difficult to reduce the cost of the printing apparatus because the memory of the printing apparatus is increased in size. In addition, an improvement in the resolution of the printing apparatus increases the size of the memory by the resolution ratio squared.
The method for a printing system, disclosed in Japanese Patent Laid-Open No. 11-98343, takes a reduction in the memory size into consideration. However, the method does not refer to a data transfer method in accordance with the bend characteristics of the output apparatus.
As described above, image forming systems suitable for high-speed electrophotographic color printers are not proposed at lower costs.
In order to resolve the above problems, an input image is divided into blocks and any positional shift of each block is corrected in units not less than pixels on the basis of an amount of correction of color shift, calculated by an amount-of-correction of color-shift calculating unit. Then, any color shift of the image that is subjected to the positional shift correction is corrected in units less than pixels and the halftoning is performed to the image that is subjected to the color shift correction to eliminate Moire patterns that can be produced in the color shift correction. However, the halftoning of the image that has been subjected to the color shift correction can cause jagged edges of the image and fine lines cannot be correctly reproduced.